Baron. Are you deficient in RAM and or Swap, if so are you experiencing excessive swapping resulting in disk thrashing? What are your RAM and Swap partition MB? If not experiencing any disk thrashing/overload due to swapping no need for it as the new USB will only be assigned as additional swap space. Better to increase physical ram and size of swap partition as actual I/O and read write is better left to Linux kernel calls.
But if the above condition are inadequate for your system see:

As long as you have at least 2-4 Gb of RAM and a hard disk greater than say, 320Gb, you should not have to use a USB for SWAP. Having a boatload of SWAP does not make a system run faster. It simply takes up space and may actually slow it down. More info on your system would help.